The driver of a vehicle prefers to drive a vehicle which has a soft ride. The driver also prefers a vehicle that is stiff in roll. The vehicle may encounter road conditions, such as bumps, that a soft ride in the suspension will accommodate, but since the vehicle is also required to turn, stiffness in the roll of a vehicle is needed. Trucks are particularly sensitive to the stiffness in the roll.
In the past, a variety of suspension systems have been interposed between the wheels and the frame of the vehicle to improve the ride. One suspension system uses leaf springs. In this type of suspension, the leaf springs, usually a plurality of leaves arranged in a stack, cannot provide both a soft ride and large roll stiffness.
One method of providing a soft ride has been to place an air bag between the axle and the frame to absorb some of the bumps. A disadvantage of using air bags is that they are very flexible in the horizontal dimension and they are very soft, that is, they have a low spring rate, which makes the vehicle soft in roll.